1. Field of the Invention
The present invention relates to a purge control system of an engine and, more particularly, to a purge control system which performs both an air-fuel ratio control and a learning purge control.
2. Description of the Related Art
A vehicle engine having a purge control system is known. Such a purge control system includes a canister which is connected between an evaporated fuel passage communicating with the interior of a fuel tank and a purge passage communicating with an intake system of the engine. Fuel in gaseous phase evaporated in the fuel tank is introduced to and sorbed (taken up and held either by absorption or adsorption) by the canister. The sorbed fuel is purged from the canister by fresh air introduced through the intake system. The purged gaseous fuel is then supplied to the engine along with the intake air. The purge control system further includes a purge valve which is disposed in the purge passage. The purge valve is turned on and off in accordance with the state of operation of the engine so as to control the rate of flow of the purge gas sucked into the engine.
The flow rate of the purge gas supplied to the engine is referred to as xe2x80x9cpurge ratexe2x80x9d. The control of the purge rate is referred to as xe2x80x9cpurge controlxe2x80x9d in the following description. A feedback control is performed to optimize the air-fuel ratio, based on an output signal from an air-fuel ratio sensor provided in the exhaust system of the engine and on a purge gas concentration which is a computed value indicative of the concentration of the evaporation fuel in the purge gas supplied to the engine. This concentration of the fuel in the purge gas is referred to as a xe2x80x9cpurge densityxe2x80x9d. In a xe2x80x9cpurge-onxe2x80x9d mode which allows the supply of the purge gas into the engine, the air-fuel ratio is controlled while being corrected with a purge-mode learned value which is obtained through a learning computation of the purge density. In contrast, in a xe2x80x9cpurge-offxe2x80x9d state which does not allow the supply of the purge gas to the engine, the air-fuel ratio is controlled while being corrected with a normally-learned value. The purge rate is controlled by the purge valve, so as to conform with a value read from a map which has been two-dimensionally formulated on two factors of engine operation: namely, engine speed and the load on the engine, and which is stored in a control means, e.g., an ECU (Electronic Control Unit).
This type of purge control system for engine is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 11-22565. In this known purge control system, a purge correction value is computed based on the purge density and the purge rate that were employed in the preceding cycle of control. Correction of the air-fuel ratio is effected using this purge correction value, in synchronization with the purge-on operation.
This known purge control system has the following disadvantages. Firstly, it is to be pointed out that this purge control system cannot stably control the purge density when the engine is operating under a light load. Fluctuation in the purge density causes a fluctuation in the air-fuel ratio of the mixture, resulting in an increase in the exhaust emissions and deterioration in the drivability. In addition, purge density is affected by a change in the air-fuel ratio that takes place when the engine is accelerated and decelerated. This serves to render the air-fuel ratio unstable, resulting in increased exhaust emissions.
Before the engine is started, evaporated fuel may stagnate in the evaporated fuel passage pipe between the full tank and the canister, even though only a small amount of fuel has been sorbed by the canister. If the engine is started with a small intake air flow rate and a small purge rate, the stagnant evaporated fuel is sucked into the engine. Consequently, a computation result indicates that the purge density is too high, and the purge rate is determined on an assumption that the purge gas is too rich. A subsequent air-fuel control, in particular under a large flow rate of the intake air, tends to make the mixture leaner, in order to compensate for the effect produced by the too rich purge gas. In this state, however, only a small amount of evaporated fuel resides in the canister and the evaporated fuel passage pipe. This small amount of evaporated fuel is rapidly depleted. Consequently, the air-fuel ratio is wrongly controlled to form an excessively lean mixture, leading to inferior drivability and increased exhaust emissions.
Another problem encountered with the use of this known purge control system is as follows. The purge control is performed by adjusting the purge rate based on the purge density. Purge rate cannot be definitely determined in a range of engine operation where the air-fuel ration feedback control and the purge learning control for computing the purge density are not performed. The feedback control is suspended during heavy-load, high-speed running of the engine with an enriched mixture, and so is the purge control.
Much evaporated fuel is accumulated in the fuel tank during long heavy-load operation of the engine as in the case of running along an ascending slope. This problem is notable particularly at high altitude where the engine power is reduced to require a greater throttle opening by pressing down on the accelerator pedal to a greater degree. Thus, driving at high altitude generally requires the engine to operate with enriched mixture for a long time. This causes a high pressure to be established in the fuel tank because purging is suspended during such long heavy-load operation of the engine, and poses a risk of escape of the evaporated fuel through a vent hole of the canister. A rapid rise in the fuel tank internal pressure may wrongly lead to warning that a fuel tank internal pressure sensor is malfunctioning, even when the sensor is operating safely.
Accordingly, an object of the present invention is to provide a purge control system of an engine which overcomes the above-described problems of the prior art.
To this end, according to the present invention, there is provided a purge control system of an engine, comprising: a canister between an evaporated fuel passage leading from the interior of a fuel tank and a purge passage leading to an intake system of the engine, for sorbing and holding evaporated fuel coming from the fuel tank and for allowing the evaporated fuel to be purged therefrom by introduction of air, so that the air and the evaporated fuel in combination forming a purge gas to be supplied to the engine; a purge valve disposed in the purge passage and opened and closed in accordance with the state of operation of the engine so as to establish a purge-on mode and a purge-off mode, thereby controlling purge rate which is the flow rate of the purge gas supplied to the engine; an air-fuel ratio sensor disposed in an exhaust system of the engine; an air-fuel ratio feedback controller for performing a feedback control of the air-fuel ratio based on an output signal from the air-fuel ratio sensor and a purge density which is computed as the concentration of the evaporated fuel in the purge gas supplied to the engine; air-fuel ratio correcting means for performing, in the purge-on mode, a purge learning control having a plurality of cycles for learning the results of computations of the purge density and for effecting correction of the air-fuel ratio based on purge learned values obtained through the learning, the air-fuel ratio correction means performing, in the purge-off mode, a normal learning control having a plurality of normal learning cycles to obtain normal learned values and effecting correction of the air-fuel ratio based on normal learned values; and controlling means for setting the number of the purge learning cycles and the number of the normal learning cycles, so as to vary the frequency of the purge learning cycles, based on the state of the computed purge density.
The controlling means may fix the number of the purge learning cycles to a predetermined value only in the first period of purge learning control which immediately follows start-up of the engine, and learn the results of the purge density computations until the total number of the purge learning cycles reaches the predetermined value.
The controlling means also may set, based on the purge density, a purge ratio which is the ratio of the purge rate to the flow rate of the intake air introduced into the engine.
The arrangement may be such that the controlling means determines, during light-load operation of the engine, a purge density correction coefficient based on the level of the load on the engine, and effects a correction of the computed purge density by using the purge density correction coefficient.
Alternatively, the arrangement may be such that the controlling means determines, when the level of the load on the engine is being changed, a purge density correction coefficient based on the amount of the change in the level of the load on the engine, and effects a correction of the computed purge density by using the purge density correction coefficient.
When the purge density correction coefficient is used, the controlling means may effect the correction of the computed purge density by using the purge density correction coefficient only in the first period of purge learning control which immediately follows start-up of the engine.
Preferably, the arrangement may be such that, when the amount of change in the purge density is greater than a purge density variation reference value while the amount of change in the purge ratio is greater than a purge ratio variation reference value, the controlling means increases the purge ratio to a target purge ratio progressively in a plurality of steps with a constant increment, whereas, when the amount of change in the purge density is smaller than the purge density variation reference value while the amount of change in the purge ratio is smaller than the purge ratio variation reference value, the controlling means controls the purge valve to open so as to increase the purge ratio to the target purge ratio in a non-stepped manner.
The arrangement also may be such that, when the feedback control of the air-fuel ratio has been suspended with the air-fuel mixture held in an enriched region, the controlling means performs the purge control by opening the purge valve, so as to achieve a constant purge ratio, a purge ratio determined based on the engine speed and the level of the load on the engine, or a purge ratio determined based only on the level of the load on the engine.
The arrangement also may be such that, when an idle switch is turned on and/or when the air-fuel ratio feedback control is suspended with the air-fuel ratio falling out of the enriched region, the controlling means performs the purge control by opening the purge valve, so as to achieve a constant purge ratio, a purge ratio determined based on the engine speed and the level of the load on the engine, or a purge ratio determined based only on the level of the load on the engine.
In accordance with the present invention having the features set forth above, the purge learned value and the normal learned value which are to be used in the correction control of the air-fuel ratio are acquired through repetition of purge learning cycles and normal learning cycles. The number of the purge learning cycles and the normal learning cycles are determined based on the purge density which is obtained through a computation. Thus, the frequency of the purge learning cycles varies in accordance with a change in the computed value of the purge density. For instance, when a large quantity of evaporated fuel remains sorbed in the canister, the purge rate, i.e., the flow rate of the purge gas, is increased, with the result that the purge density also is changed significantly. In such a case, the number or frequency of the purge learning cycles is increased so as to enable high resolution of the purge density computation. The air-fuel ratio is controlled based on the purge density that has been determined with the high resolution, whereby the rate of discharge of the exhaust emissions is stabilized. Stability of control of the exhaust emissions is impaired also by a too high density of the purge gas. In such a case, the purge control system of the invention also serves to lower the purge rate.
The above and other objects, features, and advantages of the present invention will become clear from the following description when the same is read in conjunction with the accompanying drawings.